Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce cyan, magenta, and yellow electrical signals. These signals can be transmitted to a thermal printer. To obtain a print, a black, cyan, magenta, or yellow dye-donor layer, for example, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly, which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor sheet. The thermal print head can be heated up sequentially in response to the black, cyan, magenta, or yellow signals. The process can be repeated as needed to print all colors, and a laminate or protective layer, as desired. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal transfer works by transmitting heat through the donor from the back-side to the dye-donor layer. When the dyes in the dye-donor layer are heated sufficiently, they sublime or diffuse, transferring to the adjacent dye-receiving layer of the receiver element. The density of the dye forming the image on the receiver can be affected by the amount of dye transferred, which in turn is affected by the amount of dye in the dye layer, the heat the dye layer attains, and the length of time for which the heat is maintained at any given spot on the donor layer.
At high printing speeds, considered to be 2.0 msec/line or less, the print head undergoes heat on/off cycles very rapidly. This generated heat must be driven through the dye-donor support assemblage very rapidly to effect the dye transfer from the donor to the receiver. Each layer in the donor can act as an insulator, slowing down the heat transfer through the layers of the donor to the receiver. Because of the short heat application time, any reduction in heat transfer efficiency results in a lower effective temperature in the donor layer during printing, which can result in a lower transferred dye density. It is known to overcome the low print density associated with shorter line times by increasing the printhead voltage, increasing the dye density in the dye-donor layer, or a combination thereof. Applying higher print head voltages can decrease the lifetime of the thermal print head, and requires a higher power supply, both of which increase cost. Increasing the dye density in the dye-donor layer increases costs, as well as increasing the chance of unwanted dye transfer, such as during storage of a dye-donor element.
Another problem exists with many of the dye-donor elements and receiver elements used in thermal dye transfer systems. At the high temperatures used for thermal dye transfer, many polymers used in these elements can soften and adhere to each other, resulting in sticking and tearing of the donor and receiver elements upon separation from one another after printing. Areas of the dye-donor layer other than the transferred dye can adhere to the dye image-receiving layer, causing print defects ranging from microscopic spots to sticking of the entire dye-donor layer on the receiver. This is aggravated when higher printing voltages, resulting in higher temperatures, are used in high speed printing. Another problem with high speed printing is that the more rapid physical motion of the donor/receiver assembly results in higher peel rates between the donor element and the receiver element as they are separated after printing, which can aggravate sticking of the donor and receiver.
U.S. Pat. No. 5,256,622, describes the use of several high viscosity polymers as binders in the dye-donor layer. U.S. Pat. No. 5,256,622 teaches that both ethyl cellulose ether and cellulose acetate proprionate (CAP) are equally adequate as dye-donor layer binders, as long as their intrinsic viscosity is at least 1.6. The print speeds exemplified are much slower than currently desired print speeds, which can be 2 msec per line or less. Under the slower print speeds (typically 4 msec per line or greater), both ethyl cellulose ether and CAP perform well as dye-donor layer binders.
There is a need in the art to be able to control the sensitometric curve shape of the image, which affects the density of the image formed on the receiver as a function of printing energy. This can be important in instances where existing donors need to be reformulated due to a change in the receiver composition, or perhaps due to the obsolescence of a chemical component used in the donor, forcing a reformulation of an existing donor that is manufactured for an existing population of installed printers. There is not always a way to calibrate the existing installed base of thermal printers in the trade, and changes in composition of the donor or receiver may cause an unacceptable shift in color or density of the print. These curve shape shifts are most noticeable when the color shift occurs in the neutral area. The most sensitive part of the curve is the lower scale densities around 0.15 to 0.50 status. A reflection density, referred to as the toe-region.
There is a need in the art for a means of increasing print speed while 1) maintaining or increasing print density, such as by increased dye transfer efficiency, 2) maintaining or reducing power to the print head, 3) reducing or eliminating donor-receiver sticking, and 4) controlling sensitometric curve shape.